Effects of Sitagliptin Combined with GTS-21 on Improving Glycemic Control and Diabetic Nephropathy by Anti-Apoptosis in Leprdb/db Mice

OBJECTIVE: Activation of α7 nicotinic acetylcholine receptor (α7nAChR) by GTS-21 engages the cholinergic anti-inflammatory pathway (CAP). This study investigated adding GTS-21 to sitagliptin, a DPP-4 inhibitor used to increase endogenous GLP-1, confers additional benefit over sitagliptin monotherapy on glycemic control and diabetic nephropathy (DN) in Leprdb/db mice, focusing on anti-apoptotic mechanisms. METHODS: Leptin receptor-deficient (Leprdb/db, 8-week-old) mice were randomized into control (saline), sitagliptin (20 mg/kg), or combination (sitagliptin 20 mg/kg + GTS-21 4 mg/kg) groups for 4 weeks. Oral glucose tolerance test (OGTT), serum creatinine (Scr), blood urea nitrogen (BUN), and IL-6 were measured. Renal histology was evaluated by H&E staining. Apoptosis was assessed by TUNEL staining and cleaved caspase-3 expression. In vitro, HK-2 cells exposed to high glucose (30 mM) were treated with liraglutide (100 nM) and/or GTS-21 (20 nM) to examine apoptosis. RESULTS: Combination therapy significantly improved glucose tolerance at weeks 1 and 4 (AUC reduction, P=0.0366 and P=0.0435 vs control), whereas sitagliptin alone showed no effect. Both treatments reduced Scr (P=0.0002 for combination; P=0.0101 for monotherapy), but only combination therapy attenuated renal histological injury (P=0.002) and decreased IL-6 in serum and kidney (P=0.0433 and P=0.0041). Renal apoptosis was also suppressed, with reduced TUNEL-positive cells (P=0.0043) and cleaved caspase-3 (P=0.0122). In HK-2 cells, high glucose increased cleaved caspase-3 (P=0.0324), which was attenuated by liraglutide plus GTS-21 (P=0.0320). CONCLUSIONS: Add-on GTS-21 to sitagliptin improved glycemic control and attenuated diabetic nephropathy beyond sitagliptin monotherapy in Leprdb/db mice, with reduced renal inflammation and suppressed apoptosis. These findings suggest that combined activation of GLP-1 receptor and α7nAChR signaling warrants further investigation as a renoprotective strategy in diabetic nephropathy.